LONDON — A van driving at high speed mowed down pedestrians on London Bridge late Saturday night before the occupants got out and began stabbing patrons at nearby bars and restaurants, witnesses said in an attack that police described as the latest in a string of deadly terrorist strikes to hit Britain this spring.

Witnesses described a rampage that left a trail of bloodied bodies on the bridge and in the adjacent Borough Market — both of which are London landmarks.

The low-tech but high-profile attack will raise questions about how British security services failed to stop yet another mass-casualty strike after years of thwarting such attempts.

Police said in a 4 a.m. news conference on Sunday that six people had been killed, and that officers had shot dead the three attackers. London Ambulance Service tweeted a statement saying it had taken “at least 48 patients to five hospitals across London.”

The attacks set off scenes of panic in the heart of London on a cool June evening as the city’s streets were filled with people heading home from dinner or out for a drink.

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The scene after a van hits pedestrians on the London Bridge

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Witnesses reported that a white van was traveling fast when it mounted the sidewalk and plowed into a group of people crossing the Thames River on foot.

Caption

London Bridge and nearby Borough Market were the site of chaos after a van plowed into pedestrians and attackers with knives went into the crowd. At least 12 suspects have been arrested.

June 5, 2017People attend a vigil at Potters Field Park in London to remember the victims of the attack on London Bridge and Borough Market.Hannah Mckay/Reuters

In packed pubs — normally scenes of Saturday night revelry and merriment — patrons threw chairs, bottles and glasses at the attackers as the assailants used long knives to slash their way through crowds. Tourists gaped at the carnage from the roofs of double-decker buses.

London’s Metropolitan Police said the attacks were being treated as “terrorist incidents.”

British Prime Minister Theresa May, who returned from the campaign trail to 10 Downing Street for emergency meetings with security officials, had earlier described the “terrible incidents” as “a potential act of terrorism.”

May’s Conservative Party later said it had suspended national campaigning for the general election, which is scheduled for Thursday. Other parties were expected to follow suit.

London Mayor Sadiq Khan issued a statement condemning “a deliberate and cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night.”

People in London should expect more police on the streets in the coming days, Khan said in a television interview on Sunday morning. But Londoners should not be alarmed and should not let terrorists disrupt daily life or the upcoming election, he said.

“We can’t allow them to do that,” Khan told Sky News. “We are not going to be cowered by terrorism.”

In a dawn news conference, Assistant Police Commissioner Mark Rowley said the three attackers had been shot dead and that authorities did not believe anyone else was directly involved in carrying out the carnage.

Rowley said the men had not used explosives, despite a widely distributed photo that appeared to show one of the assailants lying prone with metal canisters strapped to his body. Rowley said the vest was “a hoax.”

Saturday marks the third major attack in Britain this spring. The evening’s carnage carried grim echoes of a similar incident in late March, when a driver swerved into pedestrians at Westminster Bridge, another Thames crossing, killing four. The driver then stabbed to death a police officer at the gates of Parliament.

May had lowered the nation’s threat level only days ago — from “critical” to “severe” — after having raised it following a bombing last month at a Manchester pop music concert that was claimed by the Islamic State and that killed 22 people.

But even with the lower threat level, the nation’s intelligence services had continued to judge that another attack was likely.

Witnesses reported that a white van was traveling fast — approximately 50 mph — when it mounted the sidewalk and plowed into a group of people crossing the Thames River on foot just after 10 p.m.

The van collided with a guardrail. Bystanders said they thought the crash may have been an accident, until the occupants got out.

The three men who had been in the vehicle immediately began stabbing people on the bridge with knives before making their way to Borough Market, a foodie paradise nestled under the archways of railway viaducts that attracts locals and tourists from around the world.

It was in the market, located just south of the bridge, where police killed the attackers and ended the rampage.

“I heard many gunshots, and I heard people running away,” said Joe Dillon, 23, who was nearby when the attack occurred. “Police officers were shouting: ‘Get out of here, you need to go!’ I heard at least eight rounds of gunshots, but I’m not sure who was shooting.”

Cellphone video from a restaurant in the market showed people diving under tables amid the sound of breaking glass as officers rushed in and ordered patrons to stay down.

Tamara Alcolea, 24, who works as a bartender in a pub called Southwark Rooms, which is near the bridge, said the first indication that something was wrong was when she heard that someone had been stabbed in the proximity of London Bridge.

“Then we heard gunshots, and people started to hide beneath the tables,” Alcolea said. “We locked ourselves in the office. From the window, I could see an injured person being treated by emergency personnel. Then the police came in and told us to run. Everyone was panicking.”

As Alcolea recounted her story, she saw two friends who she had lost track of during the melee. She cried and hugged them as they reunited outside a police cordon.

Chris Jacobs, 52, and his wife Kavita Jacobs, 49, were woken up by police officers banging on their door on the third floor of an apartment building at Borough Market.

“I heard gunshots as we left the building,” said Chris Jacobs, who stood next to a petrol station outside the cordon, with no shoes on and holding his dog.

Alex Shellum, an eyewitness, told the BBC he was at the Mudlark pub in the London Bridge area when at around 10 p.m. “a woman probably in her early 20s staggered into the pub and she was bleeding heavily from the neck and from her mouth. It appeared to myself and my friends that her throat had been cut.”

Another witness, identified by the BBC as Gerard, said he saw three men running with knives: “They said, ‘This is for Allah. Then they ran up and stabbed this girl, I don’t know how many times, 10 times, maybe 15 times.”

He said he and others threw whatever objects they could find — including bottles, glasses and chairs — at the attackers in a futile attempt to stop the rampage.

Within minutes of the attack, dozens of police cars sped to London Bridge and to Borough Market, with helicopters hovering overhead. Police closed the bridge and urged the public to avoid the area.

The incident caused chaos in the heart of London in an area normally bustling on a Saturday night. Pedestrians near the bridge said they were ordered by police to run, and video footage showed people fleeing in a panic. Other images showed members of the public being escorted away from the bridge by police with hands on their heads.

Two hours after the incidents began, police were still widening cordons and pushing bystanders farther back from the scenes, as the sound of explosions — apparently controlled blasts carried out by police — echoed through the night.

British leaders scrambled to respond to the attacks.

May, a Conservative Party member who had been out campaigning ahead of an election slated for Thursday, returned to Downing Street and was being briefed by security officials.

Jeremy Corbyn, leader of the opposition Labour Party, said on Twitter: “Brutal and shocking incidents reported in London. My thoughts are with the victims and their families. Thank you to the emergency services.”

President Trump was briefed on the incident, and immediately took to Twitter to say: “We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!”

After taking criticism online for trying to use the attack to advance a policy goal that is now under review in the courts, he sent a follow-up tweet minutes later: “Whatever the United States can do to help out in London and the U. K., we will be there — WE ARE WITH YOU. GOD BLESS!”

The White House later issued a statement saying Trump and May had spoken by phone and that the U.S. president had “offered his condolences for the brutal terrorist attacks.”

A spokesman for the U.S. Department of Homeland Security said the agency was monitoring the incidents in London.

“At this time, we have no information to indicate a specific, credible terror threat in the United States,” the spokesman said.

William Booth in London and Devlin Barrett in Washington contributed to this report.